Petroleum-base compositions (e.g. crude oil and hydrocarbon compositions formed therefrom) can contain substantial amounts of petroleum waxes or paraffins. When such a normally liquid petroleum-based composition is cooled below a certain temperature referred to herein as a cloud-point temperature, the petroleum waxes or paraffins can precipitate out of the petroleum based composition forming solid particles or crystals. These solidified constituents or paraffins can then deposit on an inner surface of a pipeline resulting in a narrowing of the pipeline and restricting transport of the petroleum-based composition (e.g. crude oil during some stage of recovery, storage, transport or refining). Additionally, the paraffins can accumulate on an inner surface of a storage tank leading to a variable temperature-dependent concentration of paraffins dissolved within the petroleum-based composition. This can be disadvantageous since the accumulated paraffins can later be dissolved back into the petroleum-based composition at a high concentration when the ambient temperature exceeds the cloud-point temperature. Furthermore, the presence of dissolved paraffins can limit a temperature range for usage of petroleum-based composition (e.g. fuels such as diesel oil or gasoline, lubricants, or process feedstocks) requiring a measurement of the cloud-point temperature and some means for removing the dissolved paraffins, at least in part, to reduce the cloud-point temperature below a predetermined level.
Thus, a simple and reliable apparatus and method is needed for analyzing petroleum-based compositions and for sensing the solidification of constituents therein. Such analysis can provide information about cloud point temperature below which some of the constituents (e.g. paraffins) begin to solidify or crystallize, a pour-point temperature below which the petroleum-based composition congeals and does not readily pour or flow, and/or a freeze-point temperature at which the petroleum-based composition entirely solidifies.
Some methods for determining the cloud point for crude oils have been reviewed in an article entitled "Cloud-Point Determination for Crude Oils" by V. R. Kruka et al. (Journal of Petroleum Technology, vol 47, pp 618-687, August 1995) which is incorporated herein by reference. Briefly, these methods are based primary on visual observations, light transmission or scattering measurements, heat capacity and thermal conductivity measurements, density variation measurements, viscosity measurements.
An American Society for Testing and Materials (ASTM) standard test method based on visual observations is also disclosed in ASTM Standard D-2500 (Standard Test Method for Cloud Point of Petroleum Oils, pp. 199-201, ASTM, 1991). This visual observation method is limited to a relatively transparent oil sample; and it is based on an operator's subjective judgement of a temperature at which wax particles begin to appear in the oil sample, which is variable from one sample to the next and open to error. More objective cloud point measurements are possible with light-transmission or light-scattering measurements; but these measurements generally require a relatively transparent oil sample.
Viscosity cloud-point measurements have been based on measuring the energy required to produce sliding motion in a thin alloy steel blade by magnetostrictive excitation (see J. F. Nowell et al, Journal of Petroleum Technology, volume 8, pages 95-97, September 1956); or alternately on falling balls or rotating viscometers.
Numerous patents have also issued for determining the cloud point of oil and petroleum products, based on methods similar to those listed in the review article of Kruka et al (see U.S. Pat. Nos. 3,580,047; 3,643,492; 3,677,064; 4,519,717; 4,804,274; 4,925,314; 5,007,733; and 5,088,833).
An advantage of the apparatus and method of the present invention is that an acoustic-wave device or sensor can be used to analyze petroleum-based compositions having a wide range of viscosities and to provide information about a cloud point, a pour point, and/or a freeze point of the petroleum-based compositions.
Another advantage of the present invention is that the acoustic-wave sensor apparatus and method can be applied to analyze petroleum-based compositions in many different forms and at many different stages of oil recovery, transport, storage, processing or use; including in the form a small-volume (about 25 cm.sup.-3) sample placed within a container; in the form of a composition flowing through a pipeline during recovery, transport, or processing; or in the form of a composition contained within a tank for storage or use.
A further advantage is that the apparatus and method of the present invention can be used to provide indications of a solidification of constituents within a pipeline or tank and of an accumulation of the solidified constituents on an inner surface of the pipeline or tank.
Still another advantage of the present, invention is that the acoustic-wave sensor apparatus can be operatively connected to means for maintaining the petroleum-based composition above a predetermined point such as a cloud point, a pour point, or a freeze point.
Yet another advantage is that the apparatus and method of the present invention provides a high measurement sensitivity, thereby allowing the determination of a plurality of cloud points within a petroleum-based composition due to different constituents which solidify at different temperature.
These and other advantages of the apparatus and method of the present invention will become evident to those skilled in the art.